Burnley have confirmed defender Michael Duff will be out for four to six weeks with medial ligament damage.
The 33-year-old sustained the injury during the Clarets Carling Cup win over Barnet and was forced to miss their league win over Derby last Saturday.
"It's another injury blow for us because he had come in and performed very well," said manager Eddie Howe.
"It makes it even more disappointing."
But Howe, who has already signed six players this summer, has not finished bolstering his squad.
"I'm hoping to bring in a couple of players minimum," he said.
"That's the target. It's what I feel we need. Any more than that and it would be great but I think two players minimum."